Production of photoresist coatings

ABSTRACT

Process for the production of a resist coating, in which 
     (a) a substrate is coated with a resist composition which comprises at least one component which absorbs radiation in the near infrared region with warming of the coating; and 
     (b) the resist composition or a composition derived therefrom and obtained during the process is subjected at least once during the process to thermal treatment with the aid of radiation in the near infrared region.

The present invention relates to a process for the production of aresist coating, and to the use of this process for the production of aprimary resist coating, a solder-stop resist coating or for thesequential build-up of multilayer circuit boards.

Resist coatings are an essential tool in the production of moderncircuit boards. Inter alia, a distinction is made here between so-calledprimary resists and solder-stop resists.

Primary resists are imagewise-structured coatings on a substrate whichare intended to protect certain parts of the substrate for a temporaryperiod against the effect of a certain treatment to which the substrateis subjected, for example when a copper-laminated laminate as substrateis to be subjected to an etch treatment, in which the copper is to beremoved from certain areas of the laminate. After completion of thistreatment, the primary resists are generally removed completely from thesubstrate again. The imagewise structuring of the primary resist coatingis generally produced by imagewise exposure of a continuous layer of aresist material on the substrate, which chemically modifies the resistmaterial at the irradiated points. With the aid of suitable developers,either the irradiated or the unirradiated areas of the resist coatingcan then be dissolved and the underlying substrate uncovered. In thecase of certain resist types, for example chemically reinforced resists,it is necessary to subject the imagewise-exposed resist coating, beforedevelopment, to extended heating in order to achieve adequatedifferentiation of the solubilities of irradiated and non-irradiatedmaterial in the developer.

Solder-stop resists, which cover virtually the entire surface of aready-structured and assembled circuit board, with the exception of theareas at which contact of the printed circuit with a solder materialduring subsequent treatment of the circuit board with the soldermaterial is desired, are generally not removed after completion of thistreatment. The solder-stop resist remains on the circuit board as aprotective coating, inter alia against environmental influences and forelectrical insulation of the individual conductor tracks of the printedcircuit from one another. In addition to a photocurable componentsystem, which, as in the case of the primary resists, is employed forstructuring the resist coating by imagewise exposure, if desired heatingof the resist coating, and development, solder-stop resists frequentlyalso comprise a purely thermally curable component system, which is onlycured with the aid of heat after the structuring of the coating, andwhich improves the protective properties of the coating.

Owing to their good electrical insulation properties, solder-stop resistcompositions are also used, in particular, in the sequential build-up ofmultilayer circuit boards. In this case, a first printed circuit iscoated with a photoresist coating as insulation layer. This isstructured by imagewise exposure, if desired heating of the resistcoating, and development in such a way that holes are produced in theinsulation layer at the points at which electrical connections of thefirst printing circuit to a further printed circuit applied to theinsulation layer are later necessary. The structured resist layer isthen, if necessary, thermally cured. The holes in the insulating resistlayer are rendered electrically conductive, for example by copperplating, and the second printed circuit is then built up on theinsulation layer in a known manner. The outlined procedure is, ifdesired, repeated one or more times to give multilayer circuit boards.

Whereas “non-thermal” process steps, for example photostructuring, inphotoresist applications as mentioned above generally proceed relativelyquickly, for example within a few seconds, thermal treatment stepsrequire a significantly greater amount of time. These thermal treatmentsteps include, for example, predrying of the resist coating on thesubstrate, i.e. removal of a solvent, which frequently serves as carrierfor application of the resist compositions to the substrate, and whichgenerally requires from 20 to 30 minutes in conventional fan-assistedovens. A similar amount of time may be necessary for the above-mentionedinterim heating of the irradiated resist coating before development. Ingeneral, however, thermal final curing of solder-stop resist coatings isvery particularly time-consuming, generally requiring treatment of thecoating at temperatures in the region of about 150° C. for one hour oreven longer. Although assembly-line plants, as generally usual today incircuit-board technology, are able to conceal the time needed for thethermal treatment steps in circuit board production, they require,however, very large ovens for this purpose and/or complex plants fortransporting the assembly-line product into the ovens in order to ensurethat the boards remain in the oven for a sufficiently long time at agiven assembly line speed in order to complete the thermal treatment.

EP-A-0 115 354, Example 1, has already described the final curing of aready-structured solder-stop mask coating by means of infraredradiation, but precise details of the wavelengths of the infraredradiation used are not given. From the information on the speed at whichthe coated circuit boards are moved past the infrared radiation source,it is evident that the final curing of a conventional circuit boardtakes at least six minutes or longer.

The present invention is based on the surprising knowledge thatconventional resist compositions absorb radiation in the near infraredrange particularly well owing to the polar components present in them,during which the compositions heat up sufficiently that theabove-mentioned thermal treatment steps can be carried out within a fewseconds, typically within from 1 to 60 seconds and often in less than 10seconds. Also surprisingly, no technical disadvantages occur here, forexample solvent inclusions during predrying or increased brittlenessduring the thermal curing. In addition, a significantly better degree ofcuring is generally achieved on use of near infrared radiation for thefinal curing of solder-stop resist layers.

The invention therefore relates to a process for the production of aresist coating, in which

(a) a substrate is coated with a resist composition which comprises atleast one component which absorbs radiation in the near infrared regionwith warming of the coating; and

(b) the resist composition or a composition derived therefrom andobtained during the process is subjected at least once during theprocess to thermal treatment with the aid of radiation in the nearinfrared region.

In this application, the term radiation in the near infrared region istaken to mean, in particular, radiation having a wavelength of fromabout 760 to 1400 nm. Heating systems based on this radiation have beenknown for some time and are marketed commercially by, for example,Research Inc., US, or INDUSTRIESERVIS, DE, but have hitherto not beenemployed in resist technology and in the production of electricalcircuit boards. The radiation employed according to the inventionpreferably essentially comprises radiation having a wavelength of from760 to 999 nm.

The emitters for the near infrared radiation are preferably installed insuch a way that they irradiate the entire width of the conveyor beltpassed beneath, on which the substrates provided with the resistcoating, for example the circuit boards, are moved forwards. It may benecessary to install a plurality of emitters alongside one another. Theseparation between the emitters and the belt and the power with whichthe emitters are operated are preferably optimized as a function offurther process parameters, inter alia the specific resist composition,the thickness of the resist layer to be thermally treated, and the mostsuitable temperature for the desired thermal reaction, which is possiblefor the person skilled in the art using a few simple experiments.

There are no specific restrictions for the type of resist compositionwhich can be used in the process according to the invention, or itsconstituents. However, the application of the resist composition musthave at least one thermal process step if the advantages of the processaccording to the invention are to be realized. It must furthermore beensured that the resist composition absorbs radiation in the nearinfrared region with warming. For this reason, it should advantageouslyhave constituents containing polar functional groups which can bestimulated into thermal vibrations by the radiation in the near infraredregion. This applies to virtually all resist compositions which areusual in circuit board technology. However, the compositions to beemployed in the process according to the invention preferably compriseat least one crosslinkable compound containing (meth)acrylic or epoxidegroups.

In general, the resist compositions employed in the process according tothe invention comprise at least one photosensitive component system.“Photosensitive” for the purposes of this application means sensitive toUV and/or VIS radiation.

The photosensitive component system can be, for example, a chemicallyreinforced, positive-working system, i.e. a system which, afterimagewise exposure of the coating and thermal treatment of the layer, ismore soluble in the developer at the irradiated points than at thenon-irradiated points. Positive resists based on a photosensitivecomponent system of this type are described, for example, in EP-A-0 568827. EP-A-0 295 211 describes another type of positive resist for whoseuse the process according to the invention likewise brings considerabletime advantages, and which is also particularly suitable for thesequential build-up of multilayer circuit boards. In this positiveresist, the catalyst present in the composition for thermal crosslinkingof the composition is rendered inactive by irradiation, and thermalcuring of the imagewise-exposed resist coating is subsequently carriedout, accordingly resulting only in curing of the unexposed areas of thecoating and thus, after development, in a positive mask copy in theresist layer. The thermal curing step necessary here, which usuallyrequires from about 15 to 30 minutes, can likewise be shortened to a fewseconds by use of radiation in the near infrared region.

The process according to the invention can furthermore be used fornegative resists which comprise a photocurable or photocrosslinkablecomponent system. Unless stated otherwise, “photocurable” and“photocrosslinkable” in this application are likewise intended to meancurable or crosslinkable with the aid of UV and/or VIS radiation. Aphotocurable component system frequently employed for negative resistsis based, for example, on a free-radical photoinitiator andphotopolymerizable monomers and/or oligomers, for example vinyl or(meth)acrylate monomers and/or oligomers as crosslinkable constituent,for example diethylene glycol diacrylate, trimethylolpropane triacrylateor pentaerythritol triacrylate. Photoinitiators which can be employedhere are all conventional initiators for free-radicalphotopolymerization in the known and conventional amounts for thispurpose. If desired, co-initiators may additionally be employed. Anegative resist of this type is described, for example, in the U.S. Pat.No. 5,045,435. However, the process according to the invention is alsosuitable for cationically photopolymerizable resist types, for examplebased on onium salts as photoinitiators, for example sulfonium salts,and compounds containing more than one epoxide group per molecule. Theseresist types generally likewise have to be subjected to extended heatingat temperatures of from above 80 to 100° C. between the imagewiseexposure and development, this heating likewise requiring only a fewseconds on use according to the invention of radiation in the nearinfrared region for heating the resist layer.

Particularly suitable resist compositions for the production ofsolder-stop resists or for the sequential build-up of circuit boards arethose which comprise a photocurable component system and a thermallycurable component system. The thermally curable component system may bebased, for example, on an epoxide compound and a curing agent for thispurpose, in particular a latent curing agent, for example dicyandiamide,optionally in the presence of chlorotoluron as accelerator. Furtherresist compositions which are particularly suitable, inter alia, for theproduction of solder-stop masks are described, for example, in EP-A-0115 354 (based on organic solvents) or in EP-A-0 493 317 (based on wateras carrier).

The coating of the substrate with the resist composition can be carriedout by means of the conventional methods with which a coating can beapplied uniformly. Examples of such coating methods are spin coating,brushing, spraying, for example electrostatic spraying, reverse-rollcoating, dip coating and knife coating and the curtain-coating method.The application rate (layer thickness) and the type of substrate (layersupports) are dependent on the desired area of application. In general,the thickness of the layer before removal of the solvent should be fromabout 5 to 150 mm [sic], preferably from about 25 to 100 mm [sic], inparticular from 25 to 75 mm [sic].

If the resist composition comprises a solvent, for example an inertorganic solvent, water or a mixture of two or more than two of the saidcomponents, the solvent is preferably removed, after coating of thesubstrate, by thermal treatment of the composition with radiation in thenear infrared region with formation of the resist coating. The powerconsumption and separation of the emitters from the coated substrate arepreferably set in such a way that the predrying with formation of aresist coating on the substrate takes place for from 1 to 30 seconds, inparticular for from 3 to 10 seconds. A further time advantage of the useaccording to the invention of radiation in the near infrared regionduring predrying consists in that the substrate, owing to the shortheating time, is only heated to a temperature of from about 40 to 50°C., which means that, in contrast to conventional predrying, where thesubstrate generally has the oven temperature of from about 100 to 140°C. at the end of the predrying, any coating of the second side of thecircuit board with fresh resist composition can be begun immediatelywithout it being necessary to allow the circuit board to cool for up toseveral minutes in order to avoid exposing the circuit board to the riskof thermo-mechanical damage due to apparatus effects.

The removal of the solvent during predrying is particularly preferablycarried out with simultaneous treatment of the surface of the coatingwith a gas stream, the gas being in particular flowing air. Thetreatment of the coating with the flowing gas can be carried out, forexample, using a conventional air knife.

The predrying according to the invention with the aid of radiation inthe near infrared region for removal of the solvent is suitable both forprimary resist compositions and for solder-stop resist compositions.

After removal of solvent present in the composition, the substratecoated with the composition can be exposed imagewise in the conventionalmanner with UV/VIS radiation and then structured in accordance with thisimage with the aid of a suitable developer, as is generally conventionalin this field. The imagewise exposure can be carried out, for example,with the aid of a mask, which is preferably laid on the predried resistcoating, or by means of a laser beam, which is moved over the coating insuch a way that imagewise irradiation of the coating takes place.Heating, which may be necessary after the exposure in order to achieve asufficient solubility difference between the irradiated andnon-irradiated areas of the resist coating, is likewise preferablycarried out in accordance with the invention with the aid of radiationin the near infrared region. Heating of the coating which is sufficientfor this purpose can generally likewise be carried out for a period of afew seconds, for example for from 1 to 30 seconds, in particular forfrom 1 to 10 seconds. It may be necessary to optimize the intensity andduration of the irradiation by means of a few experiments in such a waythat any additional thermally curable component system which may bepresent in the resist is initiated during this process step to such anextent that structuring of the coating is not prevented by thesubsequent development.

If the process according to the invention is employed for theapplication of a resist which, in addition to the photocrosslinkablecomponent system, additionally comprises a purely thermallycrosslinkable component system, i.e., for example, for the applicationof a solder-stop resist, the thermally curable component system of theresist composition can be brought to reaction after the imagewisestructuring of the resist coating by exposure and, if desired, heating,as well as development.

The thermally curable component system of the resist composition ispreferably likewise cured using radiation in the near infrared region.The process according to the invention exhibits the greatest timeadvantages here compared with the conventional production of solder-stopresist coatings in which conventional thermal curing methods are used,since the usual curing time of about 1 hour can be reduced to about 5seconds, i.e. by more than 700-fold.

In a specific embodiment of the process according to the invention, thesubstrate coated with a resist composition comprising a purely thermallycurable component system is, after removal of solvent present in thecomposition by predrying, cured imagewise by means of radiation in thenear infrared region and then structured in accordance with this imagewith the aid of a developer. The imagewise curing can be carried out,for example, through a mask which is insensitive to radiation in thenear infrared region, i.e., for example, reflects the latter withoutwarming in the process, for example through a metal mask. An essentialadvantage of this embodiment of the process according to the inventionis that the resist composition does not have to comprise a photocurablecomponent system, i.e. purely thermally curable component systems can beused as resist.

The process according to the invention can be employed in all caseswhere resists, in particular photoresists, are used, in particular forthe production of primary resist coatings and solder-stop resistcoatings in the production of circuit boards and for the sequentialbuild-up of multilayer circuit boards with the aid of resists.

EXAMPLE

A resin mixture is prepared by dissolving 28.62 g oftris(2-hydroxyethyl) isocyanurate triacrylate (SR 368 from Cray Valley,Paris), 27.53 g of binder (obtained by heating 467 g of bisphenol Adiglycidyl ether having an epoxide content of from 5.26 to 5.38 val/kg,333 g of tetrabromobisphenol A, 200 g of triglycidyl isocyanurate and0.05 [lacuna] of 2-phenylimidazole at 160° C. for three hours inaccordance with EP-A-0 559 607, Example 1, and then mixing the resultantreaction product, after cooling to about 150° C., with 125 g of methylethyl ketone), 3.63 g of ethoxylated trimethylolpropane triacrylate (SR454 from Cray Valley, Paris), 0.18 g of Byk 077 (antifoam from BykChemie) and 0.01 g of hydroquinone in 16.45 g of propylene glycol methylether acetate at 45-50° C. 5.02 g of Irgacure 819 (photo-initiator basedon phosphine oxide) and 0.65 g of dicyandiamide (microfine) are added,and the mixture is homogenized using a dissolver. 17.91 g of talc aresubsequently added without further heating. After cooling to roomtemperature, the homogeneous mixture is ground.

In order to prepare a curing agent mixture, 1.56 g of2,4,6-trimercapto-s-triazine (ZISNET F from Sankyo Kasei Co. Ltd, Japan)and 0.11 g of chlorotoluron in 27.88 g of Dowanol PB 40 (mixture ofpropylene glycol methyl ether and propylene glycol butyl ether from DOW)with stirring. 0.11 g of Orasol Blue are then added, and the mixture isstirred for a further 10 minutes. After cooling to room temperature,0.33 g of the surfactant/wetting agent FC 431 (from 3M) is added.

A solder-stop composition is prepared from 100 g of the resin mixtureand 26.9 g of curing agent mixture. A copper-laminated laminate board iscoated with the composition on a K control coater model 625 K 303 unit.(Setting of the coating unit: K 202 knife-coating attachment rod No.9=125 microns; coating rate 5 m/min). The freshly coated copper laminateis left to stand in air for 20 minutes and subsequently dried at 80° C.for 30 minutes.

The copper laminate is then exposed to UV light having an energy of 1500mJ/cm² through a structured mask. The solder-stop mask is developedusing gamma-butyrolactone.

The full final curing of the solder-stop coating is carried out with theaid of a near infrared coating unit from IndustrieSerVis, Bruckmühl,Germany, within 20-28 seconds, using an irradiation system consisting ofa water-cooled near infrared heating module (120 mm×250 mm) fitted withsix high-power halogen lamps (2×2 kW outer lamps and 4×1 kW innerlamps), and the distance between the halogen lamps and the coated copperlaminate is 12 cm.

Testing of the solder-stop coating gives the following properties:

Pencil hardness (Mitsubishi pencil): 6H;

Cross-hatch class in accordance with DIN 52 151: 1 to 2;

Methylene chloride stability:

The copper laminate to be tested is immersed in methylene chloride atroom temperature.

Only after 40 minutes are the first bubbles evident on the surface ofthe solder-stop coating.

What is claimed is:
 1. A process for the production of a resist coating,in which (a) a substrate is coated with a resist composition whichcomprises at least one component that absorbs radiation in the nearinfrared region with warming of the coating; (b) said resist compositionor a composition derived therefrom and obtained during the process issubjected at least once during the process to thermal treatment with theaid of radiation in the near infrared region; and (c) the resistcomposition comprises at least one photosensitive component system.
 2. Aprocess according to claim 1, in which the composition comprises asolvent, and the solvent is removed by thermal treatment of thecomposition with radiation in the near infrared region, with formationof the resist coating.
 3. A process according to claim 2, in which theremoval of the solvent is carried out with simultaneous treatment of thesurface of the coating with flowing gas.
 4. A process according to claim3, in which the flowing gas is air.
 5. A process according to claim 1,in which the composition comprises an inert organic solvent, water or amixture of two or more than two of the said components.
 6. A processaccording to claim 1, in which the composition comprises a photocurablecomponent system and a thermally curable component system.
 7. A processaccording to claim 6, in which the substrate coated with thecomposition, after removal of any solvent present in the composition, isexposed imagewise to UV/VIS radiation and, if desired after heating, isstructured in accordance with this image with the aid of a developer. 8.A process according to claim 7, in which the heating between theimagewise exposure and the development is carried out with the aid ofradiation in the near infrared region.
 9. A process according to claim7, in which, after the imagewise structuring of the resist coating, thethermally curable component system of the resist composition is cured.10. A process according to claim 9, in which the thermally curablecomponent system of the resist composition is cured with the aid ofradiation in the near infrared region.
 11. A process according to claim1, in which the substrate coated with the composition, after removal ofany solvent present in the composition, is exposed imagewise to UV/VISradiation and, if desired after heating, is structured in accordancewith this image with the aid of a developer.
 12. A process according toclaim 11, in which the heating between the imagewise exposure and thedevelopment is carried out with the aid of radiation in the nearinfrared region.
 13. A process according to claim 1, in which the resistcomposition comprises at least one crosslinkable compound containing(meth)acrylic or epoxide groups.
 14. A process according to claim 1, inwhich the composition comprises a thermally curable component systembased on an epoxide compound.
 15. A process according to claim 1, inwhich the resist coating is a primary resist coating, a solder-stopresist coating, or a resist coating within a multilayer circuit boards.16. A process according to claim 1, in which the resist composition doesnot comprise a photocurable component system and does comprise athermally curable component system.
 17. A process according to claim 16,in which the substrate coated with the composition, after removal ofsolvent present in the composition, is exposed imagewise to UV/VISradiation and, if desired after heating, is structured in accordancewith this image with the aid of a developer.
 18. A process according toclaim 17, in which the heating between the imagewise exposure and thedevelopment is carried out with the aid of radiation in the nearinfrared region.
 19. A process according to claim 18, in which, afterthe imagewise structuring of the resist coating, the thermally curablecomponent system of the resist composition is cured.
 20. A processaccording to claim 19, in which the thermally curable component systemof the resist composition is cured with the aid of radiation in the nearinfrared region.